Ask whether there is a section of the frame that changes your sense of the portrait when viewed on its own. Look at how the subject relates to the dimensions of the frame, from the space above to the space at either side.

A portrait presented as a figure study
A portrait from this sequence
A gallery portrait with the figure as its focus
A moment to pause in the gallery
A person shown in a portrait image
A portrait alongside the other selections
A figure presented in portrait form
A portrait chosen for the gallery sequence